PROJECT SUMMARY. Immune surveillance at mucosal barriers is essential to provide an immediate defense against invasive microbes, yet must also be tightly regulated limit the potential for autoimmunity. Intraepithelial lymphocytes expressing the ?? T cell receptor (?? IEL) bridge innate and adaptive immunity, and function as a first line of defense by promoting mucosal barrier integrity. Recent reports demonstrate that basal ?? IEL function is influenced by extrinsic microbial signals. Although commensal-induced tonic type I interferon (IFN) signaling has been shown to prime mucosal innate immunity and host responsiveness to pathogen, the involvement of type I IFN in ?? IEL activation and epithelial surveillance remains unknown. Our preliminary data demonstrate that constitutive low level type I IFN signaling regulates the appropriate number and proportion of V? TCR subsets in the epithelial compartment and maintain these cells in an actively patrolling, yet immunologically quiescent state. We now show that impaired interferon ?/? receptor (IFNAR) activation induces a dysregulated ?? IEL phenotype, characterized by hyperproliferation, hypermotility and enhanced IL-4 expression. Further, we find that pathogen-associated levels of type I IFN amplify ?? IEL effector functions, including epithelial surveillance. Therefore, we propose to interrogate the mechanism by which tonic type I IFN signaling maintains ?? IEL homeostasis, whereas amplification of type I IFN in response to pathogen enhances ?? IEL effector function. In the first aim, we will take advantage of genetic models that permit the inducible ?? T-cell-specific deletion of IFNAR to examine the role of tonic IFNAR/STAT signaling in the maintaining ?? IEL homeostasis through appropriate regulation of different V? subsets. We will also investigate the mechanisms by which IFNAR signaling regulates crosstalk between different ?? IEL subsets and how this influences the proliferation, motility and effector function of these cells. Next, we will determine the functional consequence of ?? IEL dysregulation on epithelial barrier integrity under steady-state conditions. In the second aim, we will examine the mechanisms by which type I IFN amplifies ?? IEL effector function following viral infection. Using the novel intravital microscopy techniques that we pioneered and our ability to move fluidly between in vitro and in vivo models, we will investigate the molecular signals induced by pathogen-associated levels of type I IFN to enhance ?? IEL epithelial surveillance and activation. Lastly, based on the protection conferred by ?? IELs in response to enteric pathogens, we will examine the role of type I IFN-induced ?? IEL activation in the context of acute enteric viral infection. By combining, temporal and cell-specific gene targeting, cutting edge live imaging techniques, and novel models to analyze ?? IEL function ex vivo, we expect to define the molecular mechanisms by which type I IFN regulates ?? IELs under homeostatic conditions and during infection. The proposed studies will provide new insight into the molecular mechanisms that regulate ?? IEL activation and the extent to which enhanced ?? IEL effector function affects epithelial integrity and host defense.